How to show the confirmation form on any action

The simplest way to do exactly what you specify is to populate PV's using the onSuccess output and onError outputs that come pre-defined in the Write to Datastore node.  Then use an XOR gateway on the process model to send flow to one of two User Input tasks.  Make your forms to show the error or success, plus any data you got from Write to Datastore you want to show.  That and a big friendly submit button the user can use to close the form when they've looked it over enough.

The onSuccess message probably just closes the process when they submit that form.  You are free to do whatever you want to handle the error when they submit that form, or just end the process too.  Another helpful thing is a timer exception on the confirmation screens, so that it eventually submits on behalf of the user, should they navigate away or do something else like that.